A conventional SAW correlator comprises a piezoelectric substrate with an input transducer and an output transducer as shown in FIG. 1. The input transducer and the output transducer each comprise interdigitated comb electrodes (also referred to simply as interdigitated electrodes, or comb electrodes) having metal fingers which can be electrically stimulated to produce a surface acoustic wave in the piezoelectric substrate in response to an RF source voltage applied to the interdigitated comb electrodes. As an example, a 3 gigaHertz (GHz) RF signal propagating in free space will have a wavelength of 10 centimeters (cm). When coupled by an antenna into the input transducer of a SAW correlator comprising a lithium niobate piezoelectric substrate, this same 3 GHz RF signal will stimulate the SAW correlator to produce a surface acoustic wave having a wavelength of 1.16×10−4 cm which is about 100,000 times smaller than the RF signal in free space. The SAW correlator can take advantage of this wavelength compression to perform signal processing. With an appropriately coded RF signal applied to the input transducer, the SAW correlator can decode the RF signal and generate a signal at the output transducer containing the information transmitted in the RF signal. Operation of the input transducer and the output transducer can also be functionally reversed to produce a coded RF signal containing information fed into the SAW correlator in the reverse direction as a series of short pulses (i.e. feeding the information into the output transducer which now functions as an input transducer to generate an encoded RF signal at the input transducer which now functions as an output transducer).
A significant problem with SAW correlators is making the SAW correlators long physically and in code space. As the surface acoustic wave travels under the output transducer in FIG. 1, the acoustic energy in the wave is quickly dissipated as the wave generates an output voltage in the output transducer. As a result, it is difficult to build a SAW correlator with a chip length greater than about 100-200 chips.
The term “chip” is defined herein as a set of interdigitated fingers in an interdigitated comb electrode of an input transducer or an output transducer of the SAW correlator with a predetermined arrangement of spacings so as to be responsive to generate or detect a surface acoustic wave having a predetermined phase coding arrangement. The term “chip length” refers to the total number of chips required for generating or detecting a particular code pattern using the SAW correlator.
The present invention overcomes the limitations of the prior art by providing a SAW correlator which is responsive to process an electrical input signal having a chip length which can be in the range of 100 to 2000 or more.
The present invention also provides an apparatus for receiving a spread-spectrum (SS) signal transmitted at an RF frequency with information encoded with a very long chip length which can be in the range of 100 to 2000 or more.
These and other advantages of the present invention will become evident to those skilled in the art.